1. Field of Invention
This invention relates to a halogen-free resin composition containing metal hydroxide, to a method of making the resin composition and to electrical wire having this resin composition as a covering on an electrical conductor core. Such an electrical wire is useful, for example, in a motor vehicle.
2. Description of Related Art
Polyvinyl chloride has been widely used as the covering material of electrical wire for an automobile, because it is superior in properties such as mechanical strength, extrusion processability, flexibility and colouring property. However, with recent concern for the global environment, halogen-free resin material has come to be used for the production of automobile parts including the covering of electrical wires in an automobile in place of polyvinyl chloride, because polyvinyl chloride discharges a harmful halogen gas on combustion.
A halogen-free resin composition in which a metal hydroxide is blended with a polyolefin-base polymer as a flame-retardant is known as a wear resistant resin composition having the merit of no generation of a poisonous gas such as a halogen gas on combustion (see JP-A-7-176219, JP-A-7-78518 and the like). In order that such a flame-retarding resin composition has a self-extinction property, a large quantity of a metal hydroxide is required to be added; however, this causes problems that mechanical strength such as the wear resistance, tensile strength and the like of the composition are much reduced. In order to prevent the deterioration of mechanical strength, it may be considered that amounts of a polypropylene having a comparatively high hardness and a high density polyethylene are increased, but the flexibility of the covered electrical wire is reduced thereby and the processability becomes poor.
Various specific prior art proposals in this field will now be mentioned.
JP-A-6-290638 discloses resin compositions containing metal hydroxide for electrical wire insulation, in which the resin composition is based on polypropylene ( greater than 80%). Additional components are polyethylene modified with acid anhydride and styrene copolymer.
U.S. Pat. No. 5,561,185 describes resin composition for electrical wires containing metal hydroxide, in which the resin components are (a) 40-88.5% by weight of propylene which is 50% by weight or more of a ethylene/propylene random copolymer, (b) 1.5 to 30% by weight of a polyethylene modified with carboxylic acid derivative, e.g. maleic anhydride and (c) 10 to 48% by weight of an ethylene-series copolymer, typically ethylene/vinyl acetate copolymer.
U.S. Pat. No. 5,180,889 also describes a resin composition containing metal hydroxide as a covering of conductors in a crush resistant cable assembly. The resin components are (a) a low density copolymer of ethylene and alpha-olefin, (b) an elastomeric styrene-ethylene-butylene-styrene tri-block copolymer, preferably modified with maleic anhydride and (c) optionally an impact propylene and copolymer or polypropylene. Component (a) in the examples is 50% by weight or more of the total resin components.
An object of the present invention is to provide a halogen-free olefin-based resin composition comprising a mixture of selected components providing a good balance of properties, for example wear resistance, flame resistance, tensile property, flexibility and the like that are required for the covering material of an electrical wire, e.g. for an automobile.
The present invention provides a non-crosslinking and wear resistant halogen-free resin composition having good mechanical strength without loss of flexibility and being suitable for extrusion as an electrical wire covering and readily processable.
The present invention provides a resin composition containing the components:
(a) 40-90 parts by weight of a block copolymer of polypropylene segments and ethylene-propylene copolymer segments which is modified with acid anhydride and in which the polypropylene segment content is in the range 5-50% by weight,
(b) 60-10 parts by weight of a styrene-based polymeric elastomer,
wherein the total of components (a) and (b) is 100 parts by weight and no other resin component is present in the composition, and
(c) 30-200 parts by weight of a metal hydroxide based on 100 parts by weight of the components (a) and (b).
The respective components contained in the composition of the present invention are selected to provide the desired properties and are illustrated as follows.
Component (a) is an elastomeric block copolymer of polypropylene segments and ethylene-propylene copolymer segments, modified with acid anhydride, preferably 0.1 to 10% by weight of acid anhydride. The amount of the polypropylene segments in the polymer is in the range 5 to 50% by weight, preferably 15 to 45%. The polypropylene forms hard segments in the molecule while the ethylene-propylene copolymer forms relatively soft segments.
This copolymer, modified with a carboxylic acid anhydride, may be a copolymer prepared by either a graft process or a direct process.
The graft process is a process of previously copolymerizing polypropylene with ethylene-propylene copolymer and then grafting 0.1-10% by weight of an acid anhydride, by means of a peroxide or the like. The direct process is a process of copolymerizing a polypropylene with ethylene-propylene copolymer in the presence of 0.1-10% by weight of an acid anhydride.
Component (a) preferably has a melt flow rate (MFR) in the range of 0.1-5 g/10 min. MFR is an indicator of molecular length. The range 0.1-5 g/10 min is selected to achieve good cold weather performance, particularly avoidance of cracking. MFR is measured in accordance with JIS K6921-2, the entire disclosure of which is incorporated herein by reference. As described in JIS K6921-2, melt flow rate is measured using a load of 2.16 kgf at 230xc2x0 C.
The amount of component (a) is 40-90 parts by weight in the total amount of the polymers (a) and (b) of 100 parts by weight, and is preferably 60-85 parts by weight. When the proportion of the component (a) exceeds 90 parts by weight, the wear resistance of the composition is reduced. On the other hand, when it is less than 40 parts by weight the composition has reduced flexibility and becomes hard.
The styrene-based polymeric elastomer, component (b), is a polymeric elastomer containing styrene. The styrene-based elastomer is preferably a polymer obtained by block-copolymerizing styrene with butadiene, and saturating double bonds of the resulting block-copolymer by hydrogenation (known as SEBS). Typically the ratio of the styrene/butadiene is in the range {fraction (3/7)} to {fraction (2/8)} by weight. Alternatively there is used for example a styrene-based elastomer obtained by block polymerizing styrene and isoprene and hydrogenating the double bonds of the block copolymer (this product can be regarded as polystyrene-poly(ethylene-propylene)polystyrene, and is known as SEPS). The styrene-based elastomer (b) may be modified with 0.1-10% by weight of a carboxylic acid anhydride. In this case the polymer may be prepared by a graft process or a direct process.
The amount of the styrene-based elastomer (b) in the total amount of 100 parts by weight of polymers (a) and (b) in the composition is 10-60 parts by weight, and preferably 15-40 parts by weight. When the proportion of component (b) exceeds 60 parts by weight, the wear resistance of the resin composition is not improved. On the other hand, when its proportion is less than 10 parts by weight, the flexibility of the composition is poor.
The acid anhydride of component (a), and optionally also component (b) is an organic carboxylic acid anhydride, typically an unsaturated acid anhydride, preferably maleic anhydride.
Magnesium hydroxide, aluminum hydroxide and the like can be used as the metal hydroxide (c). It is preferable that the metal hydroxide particles are surface-treated with a coupling agent, particularly a silane coupling agent (for example, an aminosilane coupling agent, a vinylsilane coupling agent, an epoxysilane coupling agent, etc.) and optionally a surface-treating agent such as a higher aliphatic acid (for example, stearic acid, oleic acid, etc.) or the like. The silane coupling agent typically contains Sixe2x80x94O linkages which bond to the hydroxide. Particularly preferred is magnesium hydroxide or aluminum hydroxide surface-treated with a coupling agent, preferably a silane coupling agent, particularly an aminosilane coupling agent.
The amount of the metal hydroxide based on 100 parts by weight of the polymers (a) and (b) in the composition is 30-200 parts by weight, preferably 50-150 parts by weight and more preferably 70-100 parts by weight.
Both components (a) and (b) are selected to be halogen-free. Synthetic resin components other than components (a) and (b) are substantially, and preferably completely, absent.
Compounding agents usually included in an olefin- based resin, such as for example an oxidation inhibitor, a copper inhibitor, a lubricant and the like may be added into the resin composition of the present invention provided that the above-mentioned properties are not unacceptably deteriorated. These and other conventional additives will be readily apparent to those of ordinary skill in the art.
The resin composition of the present invention can be prepared by mixing and kneading the above-mentioned respective components by conventional methods.
The covering of an electrical wire, particularly an electrical wire for an automobile, by the resin composition of the present invention may be performed by a conventional method.
In particular, when a metal hydroxide surface-treated with an aminosilane coupling agent is used, the coupling agent bonds the metal hydroxide with the acid anhydride of the component (a) and, if present the acid anhydride of component (b). The coupling agent has a functional group reacting with the inorganic hydroxide and a functional group reacting with the organic anhydride. Also the epoxysilane and vinylsilane coupling agents have affinity to the hydroxide and the anhydride. Accordingly, the wear resistance of the resin composition is remarkably improved. Further, when an amino group exists on the lipophilic side of the silane coupling agent molecule, the reaction with polyolefin modified with an acid anhydride (i.e. component (a) and/or component (b)) advantageously suppresses the hydrophilic property of the amino group.